Intermolecular vibrations of fluorobenzene-Ar up to 130 cm-1 in the ground electronic state

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Sixteen intermolecular vibrational levels of the S0 state of the fluorobenzene-Ar van der Waals complex have been observed using dispersed fluorescence. The levels range up to -130 cm−1 in vibrational energy. The vibrational energies have been modelled using a complete set of harmonic and quartic anharmonic constants and a cubic anharmonic coupling between the stretch and long axis bend overtone that becomes near ubiquitous at higher energies. The constants predict the observed band positions with a root mean square deviation of 0.04 cm−1. The set of vibrational levels predicted by the constants, which includes unobserved bands, has been compared with the predictions of ab initio calculations, which include all vibrational levels up to 70–75 cm−1. There are small differences in energy, particularly above 60 cm−1, however, the main differences are in the assignments and are largely due to the limitations of assigning the ab initio wave-functions to a simple stretch, bend, or combination when the states are mixed by the cubic anharmonic coupling. The availability of these experimental data presents an opportunity to extend ab initio calculations to higher vibrational energies to provide an assessment of the accuracy of the calculated potential surface away from the minimum. The intermolecular modes of the fluorobenzene-Ar2 trimer complex have also been investigated by dispersed fluorescence. The dominant structure is a pair of bands with a -35 cm−1 displacement from the origin band. Based on the set of vibrational modes calculated from the fluorobenzene-Ar frequencies, they are assigned to a Fermi resonance between the symmetric stretch and symmetric short axis bend overtone. The analysis of this resonance provides a measurement of the coupling strength between the stretch and short axis bend overtone in the dimer, an interaction that is not directly observed. The coupling matrix elements determined for the fluorobenzene-Ar stretch-long axis bend overtone and stretch-short axis bend overtone couplings are remarkably similar (3.8 cm−1 cf. 3.2 cm−1). Several weak features seen in the fluorobenzene-Ar2 spectrum have also been assigned.